Such multiplexers-demultiplexers as described, then progressively perfected, especially in the French patents FR-2.543.768, FR-2.519.148, FR-2.479.981, FR-2.496.260 and in the European patent EP-0.916.963, are already known.
This latter document relates more especially to a multiplexer-demultiplexer in which the input and output fibers are positioned in the immediate vicinity of the focus of a concave mirror. Thus, divergent light received from the input are transformed by the concave mirror into parallel light beams which are directed to a plane diffraction network, i.e., grating, which sends the parallel beams back to the concave mirror and which focuses them on the ends of the output fibers.
We have endeavoured to reduce the aberrations and, especially, the spherical aberrations of such a system and it is thus that such a multiplexer-demultiplexer has been suggested, comprising a fiber carrier, an element carrying the diffraction network (i.e., grating), an intermediate element and a spherical mirror element. The fiber carrier puts the ends of the fibers in contact with the element carrying the diffraction network, the intermediate element which is in contact with the diffraction network and has the same optic index (i.e., refractive index) and the element which carries it, is terminated by a spherical surface in contact with the mirror.
This device is quite satisfactory, has enabled and still enables many embodiments.
However, in certain particular applications, the thermal instability generated by the index variation of the different elements which form the device, with the temperature and the variations of the properties, which may result, has proved detrimental.